Change Ubuntu distribution from 16.04

Thanks for the image that I’m using extensively as the base image to test all my open source dart project. I’m trying to change the ubuntu distribution used but I’m not sure I’m doing it correctly.

Currently my travis yaml file has this:

language: dart
dist: bionic

But I’m still stuck with xenial:

Operating System Details
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.6 LTS
Release:	16.04
Codename:	xenial

Is there any way to change/update the ubuntu version used?

@athomas @a14n

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